How effective are wildlife corridors like Singapore’s Eco-Link?

Man-made wildlife corridors are becoming a popular policy tool to create connectivity between natural areas for animals – but how well are they working?

Early data suggests the Eco-Link@BKE has helped some species, including the critically endangered Sunda pangolin.

More research is needed to understand which species benefit from eco corridors and why.

Despite the 28 years that have passed since the Iron Curtain fell, the Western red deer (Cervus elaphus) living on either side of the Czech Republic and Germany have never ventured beyond their respective sides.

A 2014 study done in Sumava National Park, Czech Republic, revealed that the red deer did not dare cross the places where people had once built electric fences during the Cold War. Their offspring behave the same way – deer live about 15 years on average, so no living deer would have experienced the physical barriers that have now become invisible ones.

The Eco-Link@BKE

During the 1980s, Singapore was a microcosm of development at the expense of conservation. The Bukit Timah expressway (BKE) had just been built – the fifth addition to the rapidly developing nation’s portfolio of major roads. The ten-kilometer (six-mile) expressway ran right through a strip of forest connecting the Bukit Timah Nature Reserve – which had held the last of original primary forest in Singapore – and the Central Catchment Nature Reserve. And so the lush, green weld turned into a six-lane highway capable of carrying 4,500 vehicles per hour in each direction – and separated wildlife populations just as effectively as the fences of the Cold War.

Soon after, there were increased instances of roadkill near the two nature reserves. An average of two Sunda pangolins (Manis javanica), listed both by the IUCN and in the Singapore Red Data Book as Critically Endangered, were found dead annually on these roads between 1994-2014. From 1990 onwards, several species also disappeared from the nature reserves altogether, including the large forest gecko (Gekko smithii) and cream-coloured giant squirrel (Ratufa affinis). These animals have not been sighted in Singapore since.

In a bid to mitigate these problems, the Eco-Link@BKE – an ecological corridor over the BKE – was built in 2013. It is the first overpass in Southeast Asia built specifically for wildlife, but more have been planned since. In 2011, the Malaysian government tabled a master plan to enhance existing eco corridors and establish new ones, including one within the Belum-Temengor forest landscape in Perak. These will connect the forest fragments making up the Central Forest Spine – home to the last remaining Malayan tigers (Panthera tigris jacksoni), which are classified as Critically Endangered by the IUCN. The Central Forest Spine also contributes 90% of Malaysia’s water supply.

But how effective are such projects, given that they can be considerably expensive? The Eco-Link, for example, cost $12.3 million to build.

“The purpose of the Eco-Link@BKE is to restore the ecological connection and balance between the fragmented habitats in the Central Catchment and Bukit Timah Nature Reserves, so as to allow wildlife to expand their habitat, genetic pool and survival chances,” wrote Sharon Chan, Director of Conservation at Singapore’s National Parks Board (NParks), in an e-mail. “Since the completion of the Eco-Link in 2013, NParks is very encouraged by sightings of native animals using the bridge.”

Officials have not yet released data and analysis of the Eco-Link’s usage by wildlife that is quantifiable to any meaningful extent. But NParks has set up camera traps and done nocturnal fauna surveys on the premises. These have shown Sunda pangolins, common palm civets (Paradoxurus hermaphroditus), slender squirrels (Sundasciurus tenuis), glossy horseshoe bats (Rhinolophus lepidus) and emerald doves (Chalcophaps indica) using the Eco-Link (emerald doves have been classified by Singapore’s Lee Kong Chian Natural History Museum as “uncommon residents”).

At least one Sunda pangolin crossed Eco-Link each month between October 2014 and December 2015, and no Sunda pangolins were run over near the Eco-Link between April 2015 and October 2015.

And an article published by NParks described how since the Eco-Link was built, the lesser mousedeer (Tragulus kanchil), a species critically endangered in Singapore and previously only found in the Central Catchment Nature Reserve, was seen in the Bukit Timah Nature Reserve in early 2016 – a telling sign that it had made its way there by crossing the bridge.

“As the vegetation grows taller and denser, we can expect more animals like the Malayan colugo (Galeopterus variegatus) and possibly the Raffles’ banded langur (Presbytis femoralis femoralis) to use the Eco-Link to cross between nature reserves,” Chan said. The IUCN has not assessed P. femoralis femoralis, but the entire P. femoralis species, which lives in Indonesia, the Malay Peninsula and Singapore, is listed as “near threatened.”

Landscape architecture firm Stephen Caffyn Landscape Design (SCLD) devised the Eco-Link@BKE as a 62 metre (67 yard) link bearing two metres of soil, filled with native forest species such as the tembusu (Fagraea fragrans), jelutong (Dyera costulata) and Singapore kopsia (Kopsia singapurensis). Picture an hourglass, and there’s the Eco-Link: where it is narrowest, the Eco-Link measures 50 meters (54 yards). The openings on both sides of the bridge are deliberately wide, to draw animals toward the crossing.

The Eco-Link@BKE as seen from the air. Photo credit: National Parks Board, Singapore.

The vegetation cover and thick layer of soil that enables the trees to grow to their full height are features “essential for the bridge to function,” according to the director of SCLD, Stephen Caffyn.

But Caffyn also told Mongabay that some of SCLD’s favorite ideas for the bridge design have – sadly – stayed on the drawing board. These included “funnel fencing to direct wildlife to the bridge crossing and prevent them from accessing the road” as well as “escape ramps, and hatches, and flaps in that fencing for animals already caught on the expressway or in roadside drains.”

These aspects could have made a significant impact on animals, Caffyn said, since “there are other physical elements that could serve to concentrate animals into the area and possibly cause them to cross the road instead of the [Eco-Link].”

Caffyn also explained how the bridge was designed for an unexpected set of end-users – humans.

“The signage and façade of the bridge is something that we thought would be a simple and effective way to spread the word about the function of the bridge to motorists passing below and gives it an identity that is instantly recognisable even when traveling at speed,” he said.

As Southeast Asian countries continue to develop, causing more habitats to fragment, vegetated bridges – as well as underpasses, culverts, road tunnels, arboreal webs, flyovers, and rope bridges – will become more common in the region.

“[W]e will see more ecological corridors connecting fragmented habitats,” Caffyn wrote in an e-mail. “Some of these may just be designated in order to preserve existing connections from new developments. We will also see more ecological links across roads and other barriers.”

Caffyn added that he was aware of “two or three” new proposals for other wildlife bridges in Singapore.

Roads or borders?

Thomas Lovejoy – the biologist who coined the term “biological diversity” – is known to have remarked that, “roads are the seeds of tropical forest destruction.”

Massive building projects through tropical spaces are becoming increasingly common – and not just in Singapore. The Trans-Amazonian Highway built in 1972, for example, runs 2,000 kilometres (1,200 miles) through the Amazon rainforest. The highway caused deforestation on an unforeseen scale. Yet, the Peruvian government has proposed extensions to the nation’s Transoceanic highway that would cut through Manú National Park. Described by UNESCO as having biodiversity that “exceeds that of any other place on earth,” Manú houses 850 known bird and 200 mammal species, as well as several groups of indigenous people.

The Chinese government has recently pledged hundreds of billions of dollars toward “One Belt, One Road” – a construction project involving a network of roads, bridges, railways, ports, pipelines, and power plants spanning over 60 tropical and temperate countries.

Beside a whole slate of problems that road construction can bring, including soil erosion and pollution, roads make a huge impact on biodiversity. Roads tend to break up large habitats into separate patches of land, isolating different populations of animals and reducing their genetic diversity, making extinction more likely.

Roads also allow easy access to settlers and poachers, and attract more investment, furthering the disintegration of the forest.

Eco corridors are one way to protect wildlife after roads have been built. But do they always have a positive effect?

The authors of a 2014 paper published in Conservation Biology explained that such passageways “need substantial research and planning to be successful.”

“My research at highway viaducts in Terengganu has shown that they have limited potential as wildlife crossing structures,” said Associate Professor Gopalasamy Reuben Clements of Sunway University, Malaysia, a co-author of the paper. “For example, carnivores avoided them almost completely, while only certain herbivores appeared to use them.”

It turns out eco corridors may be species-specific, though more research is needed to pinpoint why. Worse still, Clements noted that such corridors could actually be detrimental to some species.

A highway viaduct in Terengganu. Photo credit: Rimba.

“[H]uman use of some of the viaducts as camping grounds and access points into forests were relatively high, suggesting that if there are no regular law enforcement patrols, viaducts may do more harm than good for wildlife,” he said. And while rare in Singapore, poaching is a major concern elsewhere.

As the scientists wrote in the 2014 paper, “[at] best, a poorly implemented corridor is a waste of public funds; at worst, it is a consolation measure that legitimizes habitat destruction.”

The study’s authors urged government agencies to make sure there are laws and financial resources to support animals’ use of the corridors. They also suggested that officials work closely with professional ecologists on designing and constructing corridors. Designers, they said, should examine the green spaces around proposed corridor projects to identify target species, and carefully design a physical environment that is tailored to their needs.

But could there still be a downside to relying too much on such passageways – even where they have been well-researched and properly implemented?

“Malaysia still has many areas rich in biodiversity, so it is difficult to build new linear infrastructure, such as road and railway, without causing considerable environmental damage,” said Clements. “Decades of research from the Amazon to Africa have taught us that it is better not to build new roads or railway tracks in environmentally-sensitive [areas]. In certain scenarios, we need to ask ourselves: Do we really need new infrastructure? It is impossible to have constant development for economic growth without disrupting nature. Governments should plan for ‘having enough’ instead of pushing for ‘bigger and faster’.”